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How Do Lakes Die?

How do lakes die? This is a question that perhaps makes little sense without some form of context. Physically, a lake is just a function of water inputs and outputs. Just like humans require a long-term balance between calories consumed and energy burned, lakes are fed by recharge in the form of rain, surface water, and groundwater and drained by evaporation and outflow. If more water is released than is accumulated, a lake will shrink over time and eventually vanish. Note to self: avoid sentimental attachment to parking lot puddles.

But beyond drying up, can a lake die? I’ve come across some strange lakes on this blog, from Tanzania’s Lake Natron, which turns birds to stone, to Cameroon’s Lake Nyos, which burps carbon dioxide. I grant that there are lakes you should stay well away from, but what would it mean for a lake to actually ‘be dead’? Perhaps this question should be phrased in a slightly different way. Can a lake ecosystem die? An ecosystem is a community of organisms that interact with each other and their physical environment. Thus, if somehow the inhabitants of a lake were to kick the bucket, so to speak, we could safely say that the lake is no longer ‘alive’ for all intents and purposes. This wouldn’t necessarily mean that not a single living thing remained in the lake, but that the original community had been irreversibly disrupted.

Enter: eutrophication. The word stems from the Greek eutrophia, meaning healthy. A eutrophic lake is rich in phosphorus, nitrogen, and other nutrients required for organisms to flourish. What sounds on paper like an aquatic utopia actually looks something like this:

What went wrong? Somewhere between ‘rich in nutrients’ and ‘flourishing life’, we missed two words scrawled in small print on the margin of the page: ‘impending doom’. Let’s start from the beginning. Once upon a time, we had a healthy, happy lake. Streams and runoff from the surrounding fields provided a continuous source of water … and fertilizer. Fertilizer contains phosphorus, a limiting nutrient in most biological systems (it’s for precisely this reason that we fertilize food crops in the first place). As phosphorus concentrations in the lake rose, they set off something of a gold rush. All sorts of organisms began to explode, among them algae and phytoplankton. In fact, algae grew so fast that it covered the lake in a slimy green mat, commonly referred to as an algal bloom.

At this point, a couple of things began to happen. First, the bloom limited the penetration of sunlight into the water. Second, as the algae died, it fell to the lake floor and began to decay. The decay process is conducted by bacteria, which break down the organic material into carbon dioxide. In the process, bacteria require oxygen. In a balanced ecosystem, decay or decomposition is simply one component in the food web and nutrient cycling. In a eutrophic lake, the overabundance of decaying organic matter quickly drains much of the lake’s oxygen supply, a state known as hypoxia. With limited light and oxygen, it was not long before the lake became a ‘dead zone’.

A groundbreaking experiment conducted by David Schindler in the 1970s shows the impacts of phosphorus addition to a lake. The yellow curtain separates the natural water (top) from the phosphorus-enriched water (bottom)

Eutrophication has become increasingly common in lakes and coastlines across the globe. In addition to ecological impacts, it can lead to degradation of water quality and limits to recreational use of lakes, with impacts estimated at $2.2 billion annually in the US alone. In 2013, Canada’s Lake Winnipeg, one of the largest lakes in the world, was pronounced the world’s most threatened lake. Algal blooms originally triggered by human activities continue to spread based on natural processes alone, making mitigation incredibly difficult. However, for most affected lakes the answer lies in effective management of sewage treatment, fertilizer application, and land use. With proper care, eutrophication can be slowed, controlled, and reversed.

A satellite image of Lake Winnipeg

Be warned: you may have just sustained a lethal dose of mostly harmlessscience.

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